the slip-joint connection (2003) - dowec

8/8/2019 The Slip-Joint Connection (2003) - DOWEC

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The Slip-Joint ConnectionAlternative connection between pile and tower

Dutch Offshore Wind Energy Converter project

DOWEC-F1W2-JvdT-03-093/01-P

Name: Signature: Date:

Written by: J. van der Tempel TUD 2-10-2003

B. lutje Schipholt TUD

version Date No of pages

1 2-10-03 17 New Document


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Contents

1 INTRODUCTION............................................................................................................... 3

2 THE SLIP-JOINT PRINCIPLE.......................................................................................... 4

2.1 GENERAL DESCRIPTION ................................................................................................ 42.2 INSTALLATION FEATURES.............................................................................................. 52.3 DESIGN CONSIDERATIONS............................................................................................. 6

3 SLIP-JOINT APPLICATION OFFSHORE ........................................................................ 8

3.1 OVERVIEW ................................................................................................................... 83.2 VERTICAL ALIGNMENT .................................................................................................. 83.3 TOP OF THE FOUNDATION PILE ................................................................................... 103.4 CORROSION ............................................................................................................... 11

4 TRANSITION PIECE VS. SLIP-JOINT........................................................................... 12

5 CONCLUSIONS AND RECOMMENDATIONS .............................................................. 13

REFERENCES...................................................................................................................... 14

APPENDIX A. SLIP-JOINT TOWER ORIGINAL TECHNICAL DRAWING.......................... 15

APPENDIX B. SLIP-JOINT ORIGINAL INSTALLATION PROCEDURE ............................. 16


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1 Introduction

During discussions about alternative transition pieces, an idea surfaced which had beenused by the former Dutch turbine manufacturer WindMaster: the slip-joint. The principle ofthe slip-joint is to slide two conical tower sections over each other without the use of bolts,grout or welding. Should this connection be feasible for offshore use, it could mean a

reduction in offshore handling and with that, a reduction of cost.

This report explores the details of the onshore application of the slip-joint by WindMaster andthe design calculations used previously. To give a more vivid image, a WindMaster turbinewith slip-joint was visited, of which pictures have been included in chapter 2. Critical pointsfor offshore application of the slip-joint are treated in chapter 3: vertical alignment, possibledamage due to pile driving and corrosion. Chapter 4 gives a comparison of the slip-joint witha transition piece and the final chapter sums the conclusions and recommendations.


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2 The Slip-Joint Principle

2.1 General description

The Dutch company WindMaster was one of the first manufactures of wind turbines in theNetherlands. They had developed a special method of installation: the slip-joint, which will be

discussed in this chapter [1]. WindMaster went bankrupt and was taken over by LagerweyB.V. in 1998. After the takeover their innovative installation method fell into abeyance.One of the wind turbines that has been installed by WindMaster is located in Scheveningen,near the harbour. Figure 1 shows a picture of this turbine with a small but distinguishable linehalfway up the tower: the slip-joint.

Figure 1. WindMaster Turbine in Scheveningen

The slip-joint can be visualised when looking at a pile of plastic cups. The connectionbetween two plastic cups is actually a slip-joint, as can be seen in figure 2. This is the samemethod that was used by WindMaster on two tubular tower segments. In the slip-joint thetensile and compressive forces in the skin of the tubular tower are passed on through frictionforces and for a small part through contact forces between the two parts. The weight of thestructure above the joint and the conical shape of the connection cause the friction force.

Figure 2. Slip-joint connection similar to a pile of plastic cups

slip-joint


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2.2 Installation Features

The installation was done in three steps.1. The lower tower is connected to the foundation (usually a concrete slab) by means of

a bolted flange connection.2. The slip-joint, the bottom of the upper tower simply slips over the top of the lower

tower. The overlapping part is approximately 3 meters, with a diameter of 2.2 m.

3. The installation of the nacelle with its blades.

This tower was installed in 1995. The red numbers in figure 3 give an indication how much ithas sagged since the installation: far less than 5 cm in 2003.

Figure 3. View on the slip-joint from the inside. The picture to the right shows the sagging onthe top part was far less than 5 cm over 8 years

A technical drawing of the tower with the slip-joint is shown in appendix A. The drawing andfigure 3 clearly show that the cone angle is very small.The installation of the upper part of the tower has evolved over time. The first concept was toplace the upper part on top of the lower part and then pull it down further with hydraulicpumps. But when after that the nacelle was placed on the tower, the extra weight caused theupper part to slip another 20 30 cm.The new method to prevent this slipping was to lift the upper tower part over the lower partand as soon as the slip-joint made proper contact, the upper tower part was lifted again a few(6) centimetres. By releasing the upper part as quickly as possible, the connection slipsfurther, preventing unwanted slip during the rest of the installation.A copy of the original installation procedure and the justification for the drop height of 6 cm is

shown in appendix B.

Using the slip-joint instead of a conventional bolted connection saved a lot of time. Allegedly,WindMaster was able to install 4 towers with a slip-joint in the time it normally took to installjust 1 conventional tower.


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2.3 Design considerations

In this section a general overview is given of the calculation methods as applied in theWindMaster design document [1]. To arrive at the maximum design stresses, the transfer ofloads is modelled without friction. When friction is taken into account, the internal stresseswill be less.

The design method was checked by the critical German Bauamt to use this method forinstalling turbines in Germany as well.

Dimensions and ForcesThe turbine design loads are overturning moment MB and gravity load Fg.

MB Overturning momentum kNmFg Gravitational force kN

Angle of cone deg

d Average diameter of slip-joint mt Thickness m

h Height of slip-joint m

The resultant force is:

tan

g

r

FF

=

With the so-called ketelformule the stresses in the plate can be calculated:

=

dh

FP r

2t

dP

t =

Substitution gives:

tan22 =

=

th

F

th

F grt

FN

Fr

Fg

P

t

t

Mb

Fg

h

d


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The effect of frictionThe slip-joint connection is manufactured in such a way that the upper and lower towersection fit perfectly. This means that both sections make contact over the entire slip-jointsurface introducing a large area for force transfer via friction. The effect of this friction on thepreviously presented calculation of the axial stress is examined here.

Fw Friction force kN Friction coefficient -

sin cos g N W F F F = +

W NF F=

)(tancossin ++= NNNg FFFF

Which results in:

)(tan2

+

ht

Fgt

Any small value for the friction coefficient will decrease the total stress in the joint. A typicalvalue for the friction coefficient is 0.1.

Bending MomentFor the bending stress, the original calculation method assumes a linearly increasingdistribution of the contact stress over the slip-joint. Though friction will also occur, it was notincorporated in the original calculation.

13B

F h=

1max2

F P h d =

dh

MP B

=

2max

6

This results in:

th

M

t

dP Bt

=

=2max

3

2

Total StressThe total design stress, excluding friction, based on the original design document can now becalculated with:

th

M

th

FBg

t

+

=2

3

tan2

F

F1/3h h

d

FN

Fr

Fg

FW cos

FW


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3 Slip-Joint Application Offshore

3.1 Overview

The slip-joint has a large advantage over a bolted, welded or grouted connection in simplicityof installation. An ideal application offshore would be to install a foundation pile with conical

upper part on top of which the entire tower and turbine can be installed in one piece, steps 1and 2 in figure 4.

Figure 4. Installation steps for slip-joint: 1 driving the pile 2 installation of tower and turbine

Though the method seems straightforward, some details require special attention. Thetransition piece used in a standard installation procedure gives the possibility to correct anymisalignment of the foundation pile. This option is not available when a slip-joint is applied.

Foundation piles are usually straight tubular piles to prevent deformations during driving. Theconical pile head has to be strong enough to prevent deformation.

3.2 Vertical Alignment

If the foundation pile is not totally vertical, the transition piece provides the means to correctthis for the tower. Although verticality looks rather nice, it is this look that is the main reasonwhy the towers have to be in vertical position. The extra forces due to a misalignment are notdevastating. To give an indication how a wind farms with different alignment tolerances lookslike, figures 5 and 6 with 7 and 1.5 degrees of misalignment respectively are shown.

Figure 5. Impression of 7 degrees misalignment in an offshore wind farm


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It is clear that a tolerance of 7 degrees is unacceptable since the turbines do look ratherunusual when they are tilted to such extend. The 7 degrees misalignment results in a nacelleoffset of 8.59 meter.

Figure 6. Impression of 1.5 degrees misalignment in an offshore wind farm

The 1.5 degrees misalignment does show a more decent picture than the odd picture of 7degrees. It is technically feasible to keep the turbine in operation under an angle of 5degrees, although from a aesthetic point of view even this small misalignment is stillundesirable. A 1.5 degrees misalignment results in a 1.83 meter offset.

Pile driving from a jack up should ensure greater control of the pile verticality. Not with-standing this, the control will remain an issue and it is anticipated that piling would be carriedout either through a seabed template or preferably by using a vibratory hammer initiallybefore switching to a conventional hydraulic hammer. In this way it is anticipated thatvertically tolerances of the order of 0.5 degrees can be achieved.

The maximum tolerable misalignment for pile installation during for the foundations of theHorns Rev wind farm was 0.5 degrees. According to the contractor, they actually did muchbetter than this and achieved tolerances of less than 0.3 degrees offset. Figure 7 shows 2snapshots of Elsams promotion video where after a few blows of the hammer, the pilealignment is checked and found to be within bounds.

Should a slip-joint be placed on a foundation pile with an offset of 0.3 degrees this wouldresult in a 0.37 meter out of plumb position of the nacelle. This is technically not any kind of aproblem; the cone angle of the tower is even more than this so it will hardly be visible.

Figure 7. Verticality check during driving of Horns Rev foundation pile


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3.3 Top of the Foundation Pile

Hammering would damage a flange connection if it were on the foundation pile. When a slip- joint would be used, the hammering could damage the conical shape that is needed toensure a tight connection between foundation pile and tower.

The first thing that will have to be determined is what the resulting forces will be in the

foundation pile when it is driven into the seabed. The critical force is the one that will act onthe ring where the pile bends to the conical shape.

Effective HammeringIf the cone angle becomes too steep, the foundation pile cannot be driven into the seabed.The pile will absorb all the forces from the hammer and the required force to penetrate acertain depth is never achieved. This can be visualised when looking to an extreme situationsuch as a cone angle as in figure 8a.

Figure 8. Extreme cone angle (a) and realistic cone angle (b)

If this pile would be driven into the seabed, the effective force from the hammer would beabsorbed by the cone and deform it. The cone angle that is used will be between 0.4 and 0.8degrees, of which an attempt was made to visualize that in a non-technical drawing in figure8b. Such a cone angle will hardly have an effect on the drive-ability of the pile.

Eccentric LoadingA small eccentricity in the loading on the top of the cone could result in severe damage,which could make the foundation pile totally useless for a slip-joint connection. Either the topof the cone can be made very stiff to prevent distortion or special care should be taken todistribute the driving loads equally over the surface of the pile.

The latter option is achieved with one single method: using a cap that is placed on top of thepile. This is actually common use in the offshore industry. Since offshore piles vary in size,hammers are able to adjust to the pile size by attaching this kind of cap onto it.

Figure 9 illustrates the basic assembly for Vulcan Offshore Type Pile Hammers, with thecomponent parts. The pipe cap is one of these and is configured to adapt the hammers tovarious sizes of piles up to the maximum size allowed by the leaders.

This is only one example of many possible configurations of pipe caps. Others are simplyplaced on top of the pile and are made out of wood.

a b


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Figure 9. Pile Hammer with Integrated Cap

3.4 Corrosion

The turbine in figure 1 is located 100m from the North Sea. The slip-joint has no extracorrosion protection other than the layer of paint covering the entire outside and inside of thetower. According to the maintenance chief, regular inspections have shown that nosignificant corrosion has occurred over the last 7 years.For installation offshore, two options are possible:

slip-joint fully above water

slip-joint always under the waterline.The first option makes inspection easier, but places the slip-joint in or just above the splashzone where air and seawater provide a highly corrosive atmosphere. The second optionmakes inspection more complex but in that case a cathodic protection system can be usedeffectively.

Overall design and installation requirements will dictate which of these two options will beapplied. The associated effects of both options require attention but not more attention thanwhen considering grouted or bolted connections.


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4 Transition Piece vs. Slip-JointIn the DOWEC report on an alternative transition piece [2], the extreme load case waschecked for the grouted joint. Figure 10 shows a copy of the forces and moments acting onthis connection. It is assumed the configuration of a slip-joint is similar, only with a coneangle of 0.5o.

Figure 10. Extreme load case an dimensions as used for the transition piece designcalculations

Following the design calculation methods described in section 2.3, the maximum stress inthe slip-joint can be found.

)(tan2

+

=

th

Fgt =

)1.05.0(tan505.02

3484

+ = 20400 kN/m2 = 20.4 N/mm2

: 2

3 Bt M

h t

=

=05.05

5398532

= 129564 kN/m2 = 130 N/mm2

; ; :t total t F t M = + = 20.4 + 130 = 150.4 N/mm2 < 240 N/mm2

The maximum stress in this static extreme load case is below the yield stress. Note that inthis example no safety factors were incorporated. The dimensions in this calculation have notbeen altered to make comparison with the transition piece example easier. In practice, moreand more detailed design calculations must be performed to establish the optimum in coneangle and slip-joint height.

These calculations show that the slip-joint can be applied with the same dimensions as atransition piece design. Because no grout needs to be used, the costs of this material,installation of the grout and measures to transfer loads before the grout is hard enough, canbe omitted. This installation option is faster, requiring smaller weather windows and making itcheaper in comparison to other methods.

Fz= Fg 3484 kNMxy= MB 53985 kNmFxy 1009 kNd 4.3 mh 5 mt 0.05 m

0.5 degrees 0.1 [-]


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5 Conclusions and Recommendations

ConclusionsThe slip-joint has been applied onshore on several dozens of turbines without structuralreliability problems. The method shortened the installation time considerably in comparison

to a standard bolted connection.

No major problems are foreseen when the slip-joint is applied on offshore wind turbines. Apossible installation method could be to install a foundation pile with a conical upper part,cone angle < 1 degree. The nacelle and tower, with matching cone angle at the bottom-end,could be installed in one piece.The verticality of the foundation pile has to be guaranteed because the slip-joint does notallow verticality correction like the transition piece. Experts believe higher verticality demandscan be met without major cost.The structural integrity of the conical upper section of the foundation pile should stay intactfor the solution to work. It was found that the cone angle is so small that pile-driving forceswill not affect the conical part. Should this become a problem in the future, an increase in

wall thickness of the conical part will be sufficient.Eccentric loading by the hammer can be prevented by using a cap between hammer andpile, dividing the impact forces equally over the entire circumference of the pile. This iscommon practice in offshore pile driving.

RecommendationsProblems that have not been addressed are the pre-installation of ladders, j-tubes and otherexternal parts, which will normally be fitted to the transition piece.The dropping of the tower section to prevent slipping during the rest of the installation has tobe looked at closer when a detailed tower design and installation method have been workedout for offshore.The slip-joint alternative has been retrieved from the archives. This study only shows the

general description of its former onshore application and an exploration into possible futureoffshore application. It satisfies one basic requirement: reduce installation time offshore. Itwould be wise to test the correctness of the assumptions in this report in a team of experts:steel manufacturer, offshore pile driver, offshore designer, wind turbine manufacturer andcertification agency.


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References[1] WindMaster, Design document of WindMaster turbine, Archives of Lagerwey, 1989[2] Vos, A,Alternative Transition Piece, DOWEC WP2 task 2a document no.

10013000\00C0010.Vos, 2002


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Appendix A. Slip-joint tower original technical drawing


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Appendix B. Slip-joint original installation procedure


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the slip-joint connection (2003) - dowec

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